The app is gorgeously designed and super simple to navigate. Once you download, a quick tutorial gets you on your feet. Checkmark is divided into two sections: “When” and “Where.” Once you add a location under “Where” (complete with finely tuned distance settings), you can use that location to quickly remind you to take out the trash 5 minutes before you get home, send that email 15 minutes after you get to the office, etc. The “When” section is for time-based reminders. The other half of the app is for your current list of reminders, adding new ones, and seeing what you’ve completed.

Checkmark is really smart. You can set an “Arrival” or “Departure” reminder, meaning that you’ll get a push notification when you either arrive or leave a certain destination. The customization is much deeper than Apple’s Reminders, yet adding a reminder doesn’t feel overwhelming. Once you have all of your locations created (home, school, work, grocery, etc.), you’ll be able to fire through location-based reminders like a pro.

I’ve only been playing with Checkmark for a little while today, and I can already tell that it will be become my de facto reminders app on the iPhone.

You can customize the app’s geofencing accuracy in Settings based on the amount of battery life you’re willing to sacrifice (the default setting doesn’t seem to affect the iPhone’s battery very drastically at all). Overall, the Checkmark also seems way more reliable than Apple’s Reminders when it comes to setting geofences and reminding you at certain locations.

Unfortunately, location-based reminders will only work on the iPhone 4 and 4S. 3GS users need not apply. It’s also important to note that Checkmark won’t sync across your devices like Apple’s Reminders, and you can’t use it with Siri yet.